Body cells like muscle and even cells lining blood vessel or white blood cells migrating toward invading microbes exert mechanical forces. They also respond to forces: sometimes adaptively, such as exercise-induced muscular strengthening, but sometimes destructively, as in muscular dystrophies and other degenerative diseases. Our recent research has discovered that filamin A (FLNa), a major actin filament cross-linking and scaffold protein for numerous cellular components, previously shown to mediate force transmission, also contributes to force responsiveness and revealed insights into how it does so by rearrangement of specific binding sites for binding partners involved in diverse cellular functions. A novel method for detecting protein-protein interactions in cross-linked actin filament networks, Fluorescence Loss After photoconversion (FLAC), enabled this progress. We propose: 1) To obtain further insight into this mechanotrandsduction mechanism by imaging binding interactions of individual FLNa molecules under extension and compression; 2) To determine whether chemical modifications of FLNa and its partners affect their interactions under mechanical force; 3) To identify FLNa binding partners that mediate physiological and pathological mechanical responses. 4) To determine whether FLNa-based mechanotransduction quantified by FLAC to measure FLNa binding kinetics to partners accounts for the stability and persistence of diverse functions of white blood cells (migration, phagocytosis, cytokinesis, spreading on substrates of defined stiffness) and whether force presents a cryptic FLNa binding site partially buried in the cell membrane of CXCR4, an important chemokine receptor involved in white cell migration and HIV viral entry; 5) To determine whether mutant FLNa molecules associated with degenerative diseases cause defective mechanotransduction in vitro and promote apoptosis when expressed in live mechanically stressed cells.